Used in the fields of digital mobile communications and speech storage are speech coding apparatuses which compress speech information to encode with high efficiency for utilization of radio signals and recording media. Among them, the system based on a CELP (Code Excited Linear Prediction) system is carried into practice widely for the apparatuses operating at medium to low bit rates. The technology of the CELP is described in "Code-excited Linear Prediction (CELP):High-quality Speech at Very Low Bit Rates" by M. R. Schroeder and B. S. Atal, Proc. ICASSP-85, 24.1.1, pp.937-940, 1985.
In the CELP type speech coding system, speech signals are divided into predetermined frame lengths (about 5 ms to 50 ms), linear prediction of the speech signals is performed for each frame, the prediction residual (excitation vector signal) obtained by the linear prediction for each frame is encoded using an adaptive code vector and random code vector comprised of known waveforms. The adaptive code vector and random code vector are selected for use respectively from an adaptive codebook storing previously generated excitation vectors and a random codebook storing the predetermined number of pre-prepared vectors with predetermined shapes. Used as the random code vectors stored in the random codebook are, for example, random noise sequence vectors and vectors generated by arranging a few pulses at different positions.
The CELP coding apparatus performs the LPC synthesis and quantization, pitch search, random codebook search, and gain codebook search using input digital signals, and transmits the quantized LPC (L), pitch period (P), a random codebook index (S) and a gain codebook index (G) to a decoder.
However, the above-mentioned conventional speech coding apparatus needs to cope with voiced speeches, unvoiced speeches and background noises using a single type of random codebook, and therefore it is difficult to encode all the input signals with high quality.